js-data - multiple belongsTo relations using auto-incrementing primary keys, which would result in collisions, but we're using customs relation getters, so it works.

<!DOCTYPE html>
<html ng-app="app">

  <head>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/js-data/2.0.0/js-data.min.js"></script>
    <script src="script.js"></script>
  </head>

  <body>
    <h1>Open up your console to see the output</h1>
  </body>

</html>

(function() {
  var store = new JSData.DS();

  var Blog = store.defineResource({
    name: 'blog',
    relations: {
      hasMany: {
        comment: {
          localField: 'comments',
          foreignKey: 'commentableId',
          get: function(Blog, relationDef, blog, orig) {
            return store.filter('comment', {
              commentableId: blog.id,
              commentableType: 'blog'
            });
          }
        }
      }
    }
  });

  var Image = store.defineResource({
    name: 'image',
    relations: {
      hasMany: {
        comment: {
          localField: 'comments',
          foreignKey: 'commentableId',
          get: function(Image, relationDef, image, orig) {
            return store.filter('comment', {
              commentableId: image.id,
              commentableType: 'image'
            });
          }
        }
      }
    }
  });

  var Comment = store.defineResource({
    name: 'comment',
    relations: {
      belongsTo: {
        blog: {
          localField: 'image',
          localKey: 'commentableId'
        },
        image: {
          localField: 'blog',
          localKey: 'commentableId'
        }
      }
    }
  });

  var image = Image.inject({
    id: 1,
    comments: [{
        id: 1,
        commentableId: 1,
        commentableType: 'image'
      },

      {
        id: 2,
        commentableId: 1,
        commentableType: 'image'
      }
    ]
  });

  var blog = Blog.inject({
    id: 1,
    comments: [{
      id: 3,
      commentableId: 1,
      commentableType: 'blog'
    }, {
      id: 4,
      commentableId: 1,
      commentableType: 'blog'
    }]
  });

  console.log(image);
  // these comments got assigned to the image correctly!
  console.log(image.comments);

  console.log(blog);
  // these comments got assigned to the blog correctly!
  console.log(blog.comments);
})();